The swinging arm of the above type forms a leg or a part thereof in a machine that moves supported by arms. In these types of mobile devices, in which wheels are replaced with legs that are by turns lifted up and set against a support for advancement, the problem has been to control the very large force or moment present between the supporting structure and the arm in such a manner that the supporting structure will not be subjected to excess loads which are in the practice quite large and shorten the life of the bearings, keeping further in mind that the structure should not be too complicated. On the other hand there are situations where high speed and small force are needed. The present disposals of actuators can not satisfy these two opposite needs.